In the packaging of food products, it is important to detect any metal contamination for the purposes of eliminating the metal. This is done by removing, from the production stream, any packaging that contains metal. By also detecting metal in the production stream, operators are alerted to a possible failure in the machinery.
In the packaging of snack foods, product is delivered to a scale from where the product is delivered in batches to a former that leads to a packaging machine. In between the scale and the former, there are a number of chutes along which the product moves under the influence of gravity. Typically there is an upper chute and a lower chute, with the upper chute having sides that converge at a greater rate than the sides of the lower chute. As product passes from the upper chute to the lower chute, typically the product will have not only a downward directed velocity but also an angular velocity. The product also impacts against itself with the result that the product is often distributed across the entire interior of the lower chute.
Metal detectors are applied to the lower chute as it is of a much smaller diameter. However the metal detectors need to be of a substantial strength to detect any metal that may be placed towards the centre of the lower chute. Despite this, metal detectors have the disadvantage that some metal, frequently small pieces located towards the centre of the lower chute, will pass the metal detector without detection. Obviously this is a disadvantage as the metal finds its way into the food packages.
“Industrial metal detectors are used in the pharmaceutical, food, beverage, textile, garment, plastics, chemicals, lumber, and packing industries.
Contamination of food by metal shards from broken processing machinery during the manufacturing process is a major safety issue in the food industry. Metal detectors for this purpose are widely used and integrated into the production line.” (Wikipedia)
“The basic principle of operation for the common industrial metal detector is based on a three coil design. This design utilises an AM (amplitude modulated) transmitting coil and two receiving coils one on either side of the transmitter. The design and physical configuration of the receiving coils are instrumental in the ability to detect very small metal contaminates of 1 mm or smaller. Today modern metal detectors continue to utilise this configuration for the detection of tramp metal.
The coil configuration is such that it creates an opening where by the product (food, plastics, pharmaceuticals, etc.) passes through the coils. This opening or aperture allows the product to enter and exit through the three coil system producing an equal but mirrored signal on the two receiving coils. The resulting signals are summed together effectively nullifying each other.
When a metal contaminate is introduced in to the product an unequal disturbance is created. This then creates a very small electronic signal that is amplified through special electronics. The amplification produced then signals a mechanical device mounted to the conveyor system to remove the contaminated product from the production line. This process is completely automated and allows manufactures to operate uninterrupted.” (Wikipedia)